1. Field of Invention
This invention is generally related to an electric switch unit. More specifically, this invention is related to an electric switch unit for controlling air conditioning units in automotive vehicles.
2. Description of the Prior Art
A switch unit of this kind is known from the German patent application published without examination (1st publication), No. DE 38 27 090 A1. The described switch unit is comprised of a plurality of control elements to which the light is directed from a central source of light through a light-guiding member. In order to make sure that the light illuminates the symbol area of the control elements, pushbuttons for example, the light-guiding member is furnished with light-guiding bars and with projections which are associated with the individual control elements and through whose front faces the light exits and illuminates the indicating bezels of the individual pushbuttons. In order to prevent stray light from leaving from the light-guiding member and invading the spaces in between the individual control elements, an intermediate plate is provided which covers the relevant ranges of the light-guiding member. The light which leaves the front faces of said projections forms a so-called search illumination of the electric switch unit. The symbols in the indicating bezels of the control elements are to be recognized by an operator, so that the specific function of the electric switch unit is selected, for example by actuating the pushbutton. It is important that the indicating bezels/symbol areas of the control elements are illuminated in a uniform manner. In the described switch unit the light-guiding member is configured such that a hollow cylinder for the accommodation of a central source of light is formed between two light-guiding bars. Starting from this light source the light is guided through the light-guiding bars and through the projections molded to them to the control elements. The receptacle of the central light source is shaped as a hollow cylinder. Therefore, stray fields come about at the surfaces of light entry into the light-guiding bars, as a result whereof part of the quantity of light originating from the source of light is not at all emitted into anyone of the light-guiding bars. Beyond this, the stray fields coming about will lead to the creation of parasitic light which must be cancelled by countermeasures; for example by covering the light-guiding member using an intermediate plate, as described above.
Another light-guiding element is known from the German patent application published without examination (1st publication), No. 31 23 369 A1, which presents at its one side a plurality of reflecting surfaces at which the entering light rays are deviated in such a way that rays of identical intensity leave at the light exit surface. The reflecting surface is arched and consists of a plurality of individual deviating surfaces.
From the printed and examined patent application DE AS 28 25 833 a switch pushbutton is known for uniform illumination of the indicating area independently of the position of the illuminating light emitting diode. A body of transparent material is provided which has the shape of a truncated pyramid and whose convexly arched surface faces the light emitting diode. The pyramid body constitutes a self-contained component part which is inserted into the key button. Where indicating areas of a plurality of key buttons are to be illuminated, then such a pyramid body will be required for each single key button.
From the German patent application published without examination (1st publication), No. DE 32 37 589 A1, another illuminated key button is known in which the recognizability of the symbols positioned at the button is improved in that the light of a comparatively weak light source is directed on a partial range of the key surface only. For this purpose a frosted glass plate contained in the key surface is illuminated with a converging light beam with the aid of a focusing lens. It is of essential importance that the light is directed onto the frosted glass plate as a focused beam. To this end, the focal length of the focusing lens is sized such that a converging beam of light is produced. Similarly as in the above-described design it will be necessary also in this instance that each button is furnished with such an arrangement.